Bearing-mounted plastic wheel

ABSTRACT

An annular wheel of plastic is rotatably mounted on a central bearing. The wheel has a generally H-shaped cross-section, with radially outward and radially inward arms joined by a radial crossbar. An array of stiffening ribs at various circumferential locations around the crossbar of the H-shaped cross-section stiffen the wheel. To prevent the peripheral travel surface of the outward arm of the wheel from developing undulations during cooling following initial molding of the plastic wheel, the ribs have recesses in their opposite, axially outwardly directed sides. Dimensional relationships between the axial width and circumferential thickness of the rib, and the peripheral surface of the wheel and the thickness of the crossbar of the wheel are disclosed.

BACKGROUND OF THE INVENTION

The present invention relates to an annular plastic wheel mounted on abearing, and particularly to means which stiffen such a wheel, yetenable it to have a smooth, non-undulated outer peripheral travelsurface over which a belt, or the like, is trained.

The annular wheel rotates around a stationary hub, and an annularbearing is disposed between the rotating wheel and the hub. Tostrengthen the plastic wheel, radially extending stiffening ribs aredisposed at spaced intervals around the wheel. The width of the ribs inthe axial or width direction of the wheel is typically less than thewidth of the peripheral travel surface of the wheel.

Plastic wheels of this type are well known. They are used, for instance,as tensioning rollers in various fields. Over time, increasingly greaterdemands are being made, particularly on the peripheral travel surfacesof the wheels. At the circumferential locations around the travelsurface at which the radial ribs are located, undulations are producedupon manufacture of the wheels, particularly due to the presence of thestiffening ribs. The undulating surfaces can have negative effects on asystem provided with the tensioning rolls, particularly when the wheelsrotate at high speeds.

SUMMARY OF THE INVENTION

The object of the present invention is to provide a plastic wheel whosetravel surface does not become undulating during its manufacture due tothe presence of the radially extending stiffening ribs around the wheel.

The annular plastic wheel, according to the invention, has a generalH-shaped cross-section defined by an outward arm, the radially outwardsurface of which is the travel surface of the wheel, an oppositeradially inward arm which rides on or is the outer race of the bearingand a crossbar of narrower axial width which joins the inward andoutward arms. A plurality of stiffening ribs each extend radiallybetween the outer and inner arms and each also projects axiallyoutwardly in both axial directions from the crossbar of the H-shapedwheel. Each rib is generally rectangular in its cross-section.

According to the invention, each stiffening rib has two oppositerecesses defined in it, each extending axially inwardly from its axiallyoutwardly facing sides, and the recesses are provided in each rib atleast in the region of the outward arm which carries the travel surface.The recesses narrow each rib axially of the wheel, but not to be asnarrow as the crossbar of the H-shaped wheel. In the preferred form,each recess is approximately V-shaped, with an apex at the deepest partof the recess and the apex being rounded.

The stiffening ribs do not extend out to the entire axial width of thetravel surface of the plastic wheel as in known plastic wheels, but onlyover a certain lesser portion of that width. These recesses produce acertain elasticity in the travel surface. As a result, the undulationsin that surface can be practically completely avoided. The shape of thestiffening ribs is adapted to the shape of the main body of the wheel sothat upon manufacture of the plastic wheel by, for instance, injectionmolding, the ribs do not prevent uniform shrinkage but, insteadparticipate in the shrinkage movement of the main body. Therefore, nodeformations are produced on the travel surface within thecircumferential regions of the ribs. Additional machining of the travelsurface can, therefore, be dispensed with.

Due to the approximately symmetrical shape of the plastic wheel whichresults, this embodiment retains high precision upon injection molding.

The shape of the recesses in each rib are most favorable if the axialwidth of the rib at the depth of the recess amounts to between 20% and60% of the axial width of the belt travel surface and if the thicknessof the rib in the circumferential direction of the wheel corresponds to50 to 150% of the axial thickness of the crossbar of the H-shaped wheel.

More stable development of the plastic wheel is obtained by the use ofstiffening ribs. But, a smooth travel surface of the wheel is obtainedas a result of the special development of the recesses in each rib.Within the region of the crossbar of the H-shaped wheel, the travelsurface is round, since there is uniform distribution of the material.The crossbar of the wheel also has a stabilizing effect in the sections,i.e. arms of the H-shaped wheel adjoining the crossbar it on both sides.For this reason, the stiffening ribs do not cause any undulations here.Undulations would occur, if at all, only in more remote regions. Here,however, the recesses in the ribs are present, for which reason a smoothtravel surface is obtained here also. The same applies, of course, inthe regions in which no stiffening ribs are provided any longer.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention is explained further with reference to the drawings.

FIG. 1 is a partial cross-section through a plastic wheel arranged on abearing; and

FIG. 2 is an axial top view of the embodiment of FIG. 1.

DESCRIPTION OF A PREFERRED EMBODIMENT

The illustrated plastic wheel has an approximately H-shapedcross-section, including a crossbar 4 connecting the radially outwardarm 3 with the radially inward arm 5 of the wheel. U-shaped annularspaces 1 are defined on both sides of the crossbar 4. At the crossbar 4,radially extending, rectangular cross-section stiffening ribs 2 areprovided.

In order that a smooth outer peripheral travel surface of the arm 3 ofthe wheel may be obtained, rounded axially extending recesses 6 areprovided at both opposite axial sides of each rib. Each recess has anapproximately V shape. In this way, uniform shrinkage of the plasticwheel is obtained during molding, both in the regions of the stiffeningribs 2 and remote therefrom, whereby undulations in the travel surfaceare avoided.

Preferred dimensions for the rib 2 and the recesses 6 are now described.The axial width of the rib 2 is preferably less than the axial width ofthe peripheral travel surface of the radially outward arm 3. TheV-shaped recesses 6 in the rib give the rib a minimum axial width A atthe bottoms of the recesses which is greater than the uniform axialwidth C of the crossbar 4 of the H-shaped wheel. The axial width A ofthe rib is in the rangee of 20-60% of the axial width of the peripheraltravel surface of the outward arm 3. The V-shaped recesses have a radialheight at the outer surface of the rib that occupies almost the entireradial height of the rib. Furthermore, the thickness D of the rib in thecircumferential direction of the wheel (FIG. 2) is in the range of 50%to 150% of the axial width or thickness C of the crossbar 4.Additionally, the circumferential thickness D of the rib 2 is less thanthe axial thickness of the radially outward arm 3 of the wheel. Thesevarious optimum dimension assure that upon injection molding and uponcooling of the wheel following the molding, the peripheral travelsurface of the wheel does not develop undulations.

The plastic wheel can be used, for instance, as a tensioning wheel or atravel wheel. The wheel is shown mounted here on an anti-frictionbearing 7 over a hub. The inward arm 5 of the wheel is at or defines theouter race of the bearing 7. Even at high speeds of rotation of thewheel on its bearing, the inventive design of the wheel assures avibration-free, low-noise guidance of a drive belt (not shown) trainedover the peripheral surface of the wheel. As a result, the lives of thebelts and of the anti-friction bearings of the wheel are increased.

Although the present invention has been described in connection with apreferred embodiment thereof, many variations and modifications will nowbecome apparent to those skilled in the art. It is preferred, therefore,that the present invention be limited not by the specific disclosureherein, but only by the appended claims.

What is claimed is:
 1. A rotatable plastic wheel for being mounted on acentral bearing, comprising:an annular ring wheel having a generallyH-shaped cross-section, including a radially inward arm, a radiallyoutward arm with a radially outward peripheral travel surface, and aradially extending crossbar between the inward and the outward arms andthe crossbar being of narrower axial width than either arm; a pluralityof radially extending wheel stiffening ribs disposed between the inwardand the outward arms at the crossbar and the ribs being spaced atcircumferential intervals circumferentially around the crossbar; arespective recess being defined in each rib at both opposite, axiallyoutwardly facing sides of the rib, at least in the region of the outwardarm.
 2. The wheel of claim 1, wherein the axial width of each rib isless than the axial width of the travel surface of the outward arm. 3.The wheel of claim 2, wherein each recess on each side of each rib isapproximately V-shaped.
 4. The wheel of claim 3, wherein the recess hasa radial height along the rib which occupies almost the entire height ofthe rib.
 5. The wheel of claim 3, wherein each V-shaped recess has anaxially inward apex which is rounded.
 6. The wheel of claim 3, whereinthe thickness of each rib in the circumferential direction of the wheelis in the range between 50% to 150% of the axial thickness of thecrossbar.
 7. The wheel of claim 2, wherein the axial width of the rib,measured at the bottom of both recesses at opposite sides thereof, is inthe range between 20% and 60% of the axial width of the outward surfaceof the radially outward arm.
 8. The wheel of claim 7, wherein eachrecess on each of the opposite sides of each rib is approximatelyV-shaped.
 9. The wheel of claim 7, wherein the thickness of each rib inthe circumferential direction of the wheel is less than the radialthickness of the radially outward arm.
 10. The wheel of claim 7, whereinthe thickness of each rib in the circumferential direction of the wheelis in the range between 50% to 150% of the axial thickness of thecrossbar.
 11. The wheel of claim 10, wherein each recess on each side ofeach rib is approximately V-shaped.
 12. The wheel of claim 11, whereinthe recess has a radial height along the rib which occupies almost theentire height of the rib.
 13. The wheel of claim 10, wherein thethickness of each rib in the circumferential direction of the wheel isless than the radial thickness of the radially outward arm.
 14. Thewheel of claim 13, wherein each rib is rectangular in cross-section. 15.The wheel of claim 14, wherein the recess has a radial height along therib which occupies almost the entire height of the rib.
 16. The wheel ofclaim 3, further comprising an internal hub inside the inward arm and ananti-friction bearing disposed between the hub and the inward arm of thewheel.